1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED). employing a luminescent efficiency improvement layer.
2. Description of the Related Art
Organic light emitting diodes (OLEDs), which are self-emitting devices, have advantages such as a wide viewing angle, excellent contrast, quick response, high brightness, excellent driving voltage characteristics, and can provide multicolored images.
A typical OLED has a structure including a substrate, and an anode, a hole transport layer (HTL), an emissive layer (EML), an electron transport layer (ETL), and a cathode which are sequentially stacked on the substrate. In this regard, the HTL, the EML, and the ETL are generally organic thin films formed of organic compounds.
A principle of operation of the OLED having the above-described structure is as follows.
When a voltage is applied to the anode and the cathode, holes injected from the anode move to the EML via the HTL, and electrons injected from the cathode move to the EML via the ETL. The holes and electrons recombine in the EML to generate excitons. When the energy state of the excitons drop from an excited state to a ground state, light is emitted.
Luminescent efficiency of an OLED may be categorized into internal luminescent efficiency and external luminescent efficiency. Internal luminescent efficiency is influenced by how efficiently excitons are generated and converted into light in organic layers such as HTL, EML, and ETL which are interposed between a first electrode and a second electrode (i.e., between an anode and a cathode). On the other hand, external luminescent efficiency (light coupling efficiency) is influenced by how efficiently light generated in the organic layers is extracted out of the OLED. The luminescent efficiency of the OLED is reduced if the external luminescent efficiency is low, even if the luminescent efficiency in the organic layers is high (i.e., internal liminescnet efficiency is high).